But when I switch the language to French, which uses a prefix folder, I get this path for the image which is a 404
domain.tld/fr/sites/all/modules/contrib/openlayers/theme/default_dark/img/marker-red.png

The /fr/ appears to be the problem. Most likely the Javscript used in this module isn't setting up the paths right. This might also be a problem for you if your Drupal is installed in a sub-folder I'm guessing

Thank, but please don't be mad on me, i never applied patches
could you guide me in few lines how to do it ?
i actually could follow the diffs by if there some better way i would like to know..
and thanks again...